1. Field of the Invention
This invention relates to an apparatus for controlling the transportation of printed materials, such as documents, manuscripts, printed sheets or the like sheet media, in a printer, an image reader, an information processing machine and the like.
2. Description of the Related Art
In a printer in which printed materials or paper sheets (hereinafter referred to as "sheets") are transported to a printing section to be printed, or in an image reader in which sheets are transported to an image reading section where data on the sheets are read, a plurality of rollers are provided to transport the sheets and these rollers are arranged in such a manner that the sheet is always in contact with at least one roller. In general, these rollers are driven or rotated by a single motor in order to reduce the cost.
In such a sheet transporting apparatus, the number of rollers which directly transport the sheet, i.e., the number of rollers which are in contact with the sheet changes in accordance with the position of the sheet. Therefore, if the driving capacity or torque of the motor is relatively small, and the load exerted on the motor changes according to the number of rollers which directly contact the sheet, the sheet transporting speed will also change.
Thus, an apparatus, in which a change in the sheet transporting speed due to the above-mentioned causes is prevented, has long been required so that print quality or reading accuracy can be maintained.
In FIGS. 5a and 5b, a printer or image reader with a conventional sheet transporting apparatus, as known in the prior art, is shown in cross-section. The sheet transporting apparatus includes a pair of rollers 1 and 1' and a pair of rollers 2 and 2' having, each having a same diameter, for transporting a sheet 4 and a head 5 for scanning the sheet in a line direction (i.e., a direction perpendicular to the drawing) to carry out a printing or image reading operation.
As shown in FIG. 5a and FIG. 5b, each of the two pairs of rollers includes a drive roller 1 or 2 and a pinch roller 1' or 2' arranged opposite to the drive roller. The two drive rollers 1 and 2 have the same diameter and are operatively connected to a motor 3 by means of a rotating shaft of a belt 6. When the motor 3 rotates, the two drive rollers 1 and 2 rotate at the same speed. Thus, the sheet 4 is first held and transported by the pair of rollers 1 and 1' and then by the pair of rollers 2, 2', in turn, at a predetermined speed.
The sheet which is fed to and held by the rollers 1 and 1' is, (1) transported toward the head 4 by only one pair of rollers 1 and 1', then (2) transported by two pairs of rollers 1, 1' and 2, 2', and finally (3) transported only by a single pair of rollers 2 and 2' and discharged. During the transportation, the sheet is scanned and printed (or read) by the head 5.
It is preferable that the motor 3 is a pulse motor or a direct current motor which can be driven at a certain exact speed on the basis of an instruction.
In the above-mentioned conventional sheet transporting apparatus, the relationship between the sheet transporting speed V1 when the sheet is held by and transported only by the rollers 1 and 1' (or rollers 2 and 2') and the sheet transporting speed V2 when the sheet is held by and transported by both pairs of the rollers 1, 1' and 2, 2' is defined, in general, as follows. EQU V1.gtoreq.V2
The reason is that the load on the motor increases if the number of rollers which hold the sheet increases. Thus, the motor 3 will not be able to rotate at a speed in accordance with an instruction, as shown in FIG. 6 in which a variation in transportation speed is shown. At a certain speed, if the sheet is progressively held and transported by roller 1, roller 2, roller 3 and so forth, the sheet speed will be reduced one in steps. Such a phenomenon has been confirmed by experiment and experience.
Therefore, if V1 is larger than V2 (V1&gt;V2), the printing pitch or the reading pitch will change during a printing or reading operation. Thus, the printing or reading quality will be reduced. If the driving capacity of the motor is increased, such problems will be solved. However, according to such a solution, a motor will become more expensive. Also, if a larger motor is used, the noise from the motor will increase.
If the speed of the sheet is continuously monitored and the motor is feed-back controlled in accordance with the monitored speed, a speed monitoring encoder or the like will be necessary and therefore it will cause an increase in the cost, although such a drive mechanism is required to be as simple as possible.